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The Doctor, Jamie and Zoe have landed on a moving dome on the planet Mercury. While the Doctor and Zoe work out what the dome is, the TARDIS slides off the back of the dome. Before they can retrieve it, they are joined by Alison Kolitz, who thinks they are here to help.

Inside the dome, which is a scientific base, Alison explains that one day she found a shiny silver creature outside, on the dark side of the planet, and brought it in. As it warmed up, it started attacking the crew. One scientist, Oleg, seems to be unaware of what's happening around him, and just mumbles about "giants". The Doctor thinks that if they speed up the base to the dark side of the planet, the creature might become incapacitated. Another scientist, Chelo Chascarillo, contacts them from another room and is given the task of overriding the thermostats.

Wayne Songo'o, another scientist, bursts into the room and tells them that, while he was eating lunch with Guto, Guto was attacked by the creature. Guto then left the station through the airlock, without taking a space suit. Meanwhile, Chelo, watched onscreen by the others, heads for the thermostat controls, but is followed by the creature. After it attacks her, Wayne is assigned her task, and the Doctor and Jamie will act as a diversion.

Zoe accompanies Wayne, Alison gets Oleg into a space suit, and the Doctor and Jamie run from the creature into the room in which Chelo is sitting. As they get her into a space suit, the creature is right outside the door, looking in. Chelo begins speaking of "giants".

On the top of the dome, Zoe and Wayne see dozens of the creatures in the path of the station. One of them reaches the station and starts to flatten itself on the station. Inside, however, Chelo begins talking, and the Doctor realises the creature is talking through her.

The creature is a Mercurial. They live on the light side of the planet, but after the dome was built, they were attacked by "giants". This Mercurial headed to the dark side to warn the humans, but the extreme cold weakened it. When Alison brought it into the station, it tried to communicate, first through Oleg, then Guto, and finally, successfully, through Chelo.

The Doctor's plan is for the Mercurials to cover the dome and reflect power from the solar pylons onto the "giants". As they put the plan into action, however, the Doctor notes the Mercurials' reaction to the solar pylons, and realises that the pylons are the "giants".

With the threat over, the Mercurials retrieve the TARDIS and the Doctor and his friends leave. The Mercurials head back to the light side, and the Sunwatcher crew return to work.